1. Field of the Invention
The present invention relates to an electron emission element and a method of manufacturing the same, and more particularly, to an electron emission element having a plurality of electrodes each having a conical portion, an insulating layer having openings centered on conical portions, and a deriving electrode, at least, part of which is formed near conical portions, and a method of manufacturing the electron emission element.
2. Related Background Art
Hot cathode electron emission elements have been frequently utilized as conventional electron emission sources. Electron emission utilizing hot electrodes has large energy loss by heating, and preheating is undesirably required.
In order to solve these problems, several cold cathode electron emission elements have been proposed. Of these elements, a field effect electron emission element for emitting electrons by electric field emission is available.
A typical example of the field effect electron emission element is shown in a partial sectional view of FIG. 1, and steps in manufacturing this electron emission element are shown in FIGS. 2A to 2D.
As shown in FIG. 1, each conical electrode 19 made of Mo (molybdenum) or the like is formed on a substrate 21 of, e.g., silicon. An insulating layer 20 such as an SiO.sub.2 layer has an opening. This opening is centered on the electrode 19. A deriving electrode 18, part of which is formed near the conical portion is formed on the insulating layer 20.
In the electron emission element having the above structure, a voltage is applied between the substrate 21 and the electrode 18, electrons are emitted from the conical portion having a high field intensity.
The above electron emission element is manufactured by the following steps.
As shown in FIG. 2A, the insulating layer 20 as an oxide film (e.g., an SiO.sub.2 film) is formed on the substrate 21 of, e.g., Si. The Mo layer 18 is formed by electron beam epitaxy, and an electron beam resist such as PMMA (polymethyl methacrylate) is spin-coated on the Mo layer 18. The resist film is irradiated with an electron beam and is patterned. The resist is partially removed with isopropyl alcohol or the like, thereby selectively etching the Mo layer 18 and hence forming a first opening 22. After the electron beam resist is completely removed, hydrofluoric acid is used to etch the insulating layer 20, thereby forming a second opening 23.
As shown in FIG. 2B, the substrate 21 is slightly inclined by an angle .theta. while being rotated about an axis X, and an Al layer 24 is formed on the upper surface of the Mo layer 18. In this case, aluminum is also deposited on the side surface of the Mo layer 18. By controlling the deposition rate of aluminum, the diameter of the first opening 22 can be arbitrarily reduced.
As shown in FIG. 2C, Mo is vertically deposited by electron beam epitaxy on the substrate 21. In this case, molybdenum is deposited on the Al layer 24 and the substrate 21 as well as the side surface of the Al layer 24. The diameter of the first opening 22 can be gradually reduced when deposition of the Mo layer progresses. When the diameter of the first opening 22 is reduced, the deposition area of the metal (Mo) deposited on the substrate 21 is reduced. Therefore, a substantially conical electrode 19 is formed on the substrate 21.
Finally, as shown in FIG. 2D, by removing the deposited Mo layer 25 and the deposited Al layer 24, an electron emission element having the substantially conical electrode 19 is prepared.
In the conventional electron emission element described above, the height, the angle, and the diameter of the bottom surface of the electrode are determined by various manufacturing conditions such as the size of the first opening, the thickness of the oxide film, and the distance between the substrate and the deposition source. Therefore, reproducibility of the electrode is degraded. When a plurality of electron emission elements are simultaneously formed, variations in conical shape typically occur.